<template>
    <ul class="list">
        <li class="item" v-for="item of letters"
        :key="item"
        @click="handleLetterClick"
        @touchstart="handleTouchStart"
        @touchmove="handleTouchMove"
        @touchend="handleTouchEnd"
        :ref="item"
        >
        {{item}}
        </li>
    </ul>
</template>

<script>
    export default {
        name: "CityAlphabet",
        props:{
            cities:Object
        },
        data(){
            return{
                touchStatus : false,//定义一个标志位
                startY : 0,
                timer : null //性能优化，减少手势移动刷新动作频率
            }
        },
        updated(){
                this.startY = this.$refs['A'][0].offsetTop
        },
        computed:{
            letters() {
                const letters =[]
                for(let i in this.cities){
                //let key in obj
                    letters.push(i)
                }
                return letters
            }
        },
        methods:{
            handleLetterClick(e){
                this.$emit('change',e.target.innerText)
            },
            handleTouchStart(){
                    this.touchStatus =true
            },
            handleTouchMove(e){
                if (this.touchStatus){
                    if(this.timer){
                        clearTimeout(this.timer)
                    }
                    this.timer =  setTimeout(()=>{
                        //const startY = this.$refs['A'][0].offsetTop//57
                        const touchY = e.touches[0].clientY - 58
                        const index = Math.floor((touchY-this.startY)/22)-1
                        if(index >= 0 && index < this.letters.length){
                            this.$emit('change',this.letters[index])
                        }
                    },16)
                                    }
            },
            handleTouchEnd(){
                    this.touchStatus=false
            }
        }
    }
</script>

<style lang="stylus" scoped>
@import '../../assets/style/varibles.styl'
.list
    display flex
    flex-direction column
    justify-content center
    position: absolute
    top:1.58rem
    right:0
    bottom:0
    width:.4rem
    .item
        text-align center
        line-height .44rem
        color: $bgColor
</style>